Metal-Oxide-Semiconductor (MOS) technology has been used widely. A MOS device can work in three regions including a linear region, a saturation region, and a sub-threshold region, depending on the gate voltage Vg and the source-drain voltage Vds. The sub-threshold region is a region in which gate voltage Vg is lower than the threshold voltage Vt. A parameter known as Sub-threshold Swing (SS) represents the easiness of switching the transistor current off and on, and is a factor in determining the speed of a MOS device. The sub-threshold swing can be expressed as a function of m* kT/q, where m is a parameter related to capacitance, k is the Boltzman constant, T is the absolute temperature, and q is the magnitude of the electrical charge on an electron.
Previous studies have revealed that the sub-threshold swing of a typical MOS device has a limit of about 60 mV/decade at room temperature, which in turn sets a limit for further scaling of operational voltage VDD and threshold voltage Vt. This limitation is due to the diffusion transport mechanism of carriers. For this reason, existing MOS devices typically cannot switch faster than 60 mV/decade at room temperatures. With such a limit, faster switching at low operational voltages is difficult to achieve. To solve the above-discussed problem, Tunnel Field-Effect Transistors (TFETs) have been explored. In a TFET, electron injection is governed by the band-to-band tunneling from the valence band of the source to the conduction band of the channel. Since the current mechanism is determined by tunneling, the SS can be very low at the initial stage the TFET is turned on. When the voltage increases, however, the SS quickly increases, and the current no longer increases fast enough. This posts a problem for improving TFETs.